Measurement of temperature is often important, and sometimes vital, for a particular end. While measurement devices have been fairly well developed for measuring temperatures at the location of the sensing element of the measurement device, remote temperature measuring has not heretofore been achieved, at least to the extent of making available a device and method that is practical, dependable and accurate.
The echosonde (or acoustic sounder) has been heretofore developed and has proved to be useful, particularly in geophysical research. As is known, operation of the echosonde depends upon the physical process of reflection (scattering) of sound by atmospheric or oceanic irregularities or particles. Early work in this area is reported by G. W. Gilman, H. B. Coxhead and F. H. Willis, "Reflection of Sound Signals in the Troposphere", in the Journal of Acoustic Society of America, Vol. 18, pages 274-283 (1946), and a particular form of echosonde is shown and described in U.S. Pat. No. 3,675,191. A complete description of the echosonde may also be found in "Advances in Atmospheric Acoustics" by E. H. Brown and F. F. Hall, Jr., "Review of Geophysics and Space Physics", Vol. 16, pages 47-110 (1978). In addition, an interferometric system for measuring a physical condition such as temperature by sensing medium density is described in U.S. Pat. No. 3,604,252.
However, no form of the echosonde has been found to be successful in providing capabilities for remote measurement of mean temperatures and temperature profiles in the atmosphere or oceans. An attempt has been heretofore made to combine echosonde techniques with interference fringes, but while the combined instrument produced dark bands in echosonde facsimile recordings, the positioning of the dark bands was not considered to be sufficently sensitive to temperature changes for geophysical application (E. H. Brown, C. G. Little and W. M. Wright, "Echosonde Interferometer for Atmospheric Research", Journal of Acoustic Society of America, Vol. 63, pages 694-699 (1978)).